There have been phrases floating around, such as 'W + mouse1', in reference to certain classes being frowned upon as unskilled.
In order to resolve such controversy, a strictly objective examination will be made of each individual class. In making such an examination, it is key to be as neutral as possible. It is obvious that each class requires a different mindset and is outfitted with different equipment, so how are we to remain neutral in our examination?
The most concrete commonality between all players of all classes is, of course, their interface. From the humble medic to the bombastic scout, there is a clear consensus that the most universal way of determining class complexity (and thus the necessary skill level) is by quantifying the actions that a player must perform to effectively use the class. And the universal method of inputting these actions is controls.
For the scout, the requisite controls are mouse 1, W, and spacebar. It is worthwhile to note that an additional shade of complexity results from the fact that the player must commonly tap the spacebar twice - deepening the respect that must be rendered to scouts by a small measure.
The soldier requires the exact same keys as the scout, but rather demands of the player that they can activate mouse 1 and spacebar in co-ordination.
The pyro infamously relies upon mouse 1 and W. It is often regarded as a viable practice to hold these buttons down constantly.
The demo requires mouse 1, mouse 2, Q, and S - by far, the most complex class thus encountered! Note the unusual dimension resulting from reliance upon S far more than W.
The heavy makes uses of mouse 1, mouse 2, and W. It is, however, sufficient to simply keep mouse 2 active at all times.
The engineer, the final defensive class, is astounding by comparison to all other classes but one. Not only must he use W, mouse 1, and 4, but after pressing 4 he must then press 1, 2, 3, or 4! The depth of skill necessary to master the engineer reveals itself quickly.
The medic, for all its reverence, holds a shocking secret: by merely holding down mouse 1 and sporadically using W, it is entirely possible and quite likely that a player may reach the upper echelons of the scoreboard! While the medic may be appreciated for his strategic value, the utter lack of skill required calls into question such a reputation.
The sniper is also associated with the elite, but a further shock is found: no more is needed to play this class than mastery of mouse 1, and mouse 2. Indeed, the keyboard may go unused and a phenomenal number of points can be easily racked up regardless.
Lastly, the spy: undisputedly the most complex class of them all. Mouse 1, mouse 2, q, 2, 3, and 4 are all immediately required, and similiar to the engineer, the spy must judiciously use a total of 19 more keys after pressing 4 to select a disguise! Even with his reputation for complexity, it is impossible to overstate the level of skill required by the spy.
Thus, in descendingly skill level, I present to you the classes:
Spy
Engineer
Demoman
Scout/Soldier (Tie)
Heavy
Medic/Pyro (Tie)
Sniper
By this quantifiable, objective examination, you may be able to make factual and, indeed, indisputable commentary dismissing certain classes as 'noob' while bemoaning the difficulty of other classes.